I would recommend putting it outside in partial sun in addition to the other things that you may need to do. After it is doing better you could transition it to full sun or move it back inside. My fukiens have always recovered quickly outside. I repotted one of them and accidentally cut 95% of the roots off (they were all fused together). All the leaves fell off and then it regrew another set. It took a little while but the tree is fine now.

Also, stop picking it up and putting it in the sink. Fukiens do not like to be moved around too much. It may just be dropping the leaves because it doesn't like this kind of treatment (they are kind of temperamental).